1. Field of the Invention
The present invention relates to a halftone processing method.
2. Description of the Related Art
In a conventional method for forming an image on a recording medium, halftone processing is often used in reproducing the density with a limited gradation number. Halftone processing is processing for expressing the gradation by changing the ratio of a color material adhering region.
Halftone processing includes processing for converting image data on an amplitude modulation (AM) screen (hereinafter simply referred to as “AM screen processing”) and processing for converting image data on a frequency modulation (FM) screen (hereinafter simply referred to as “FM screen processing”). In the AM screen processing, the gradation is expressed by modulating the dimension of a color material-adhering region (i.e., the size of halftone dots). In addition, in the AM screen processing, halftone dots have a characteristic shape, are oriented in a characteristic orientation (at a halftone-dot angle), and are arranged in a periodical intimate contact with one another (i.e., by periodical screen ruling).
On the other hand, in the FM screen processing, tiny dots of the same specific size are spatially arranged in a dispersed manner at random. In addition, in the FM screen processing, the gradation is expressed by the density of arrangement of tiny dots.
In forming an image by using the FM screen processing, it is necessary to stably record tiny dots. Accordingly, the AM screen processing is used by an image forming apparatus that cannot stably reproduce tiny dots. However, in using the AM screen processing, a phenomenon, such as moire on a document, broken thin lines, or jaggy may occur.
The phenomenon of document moire is a periodical pattern that is visually recognized if high frequency components of an input image interfere with periodically arranged halftone dots and if the high frequency components of the input image are folded into a low frequency region. The phenomenon of broken thin line may occur if the angle of the thin line is close to the angle of halftone dots so that the thin line and the halftone dots interfere with one another. The phenomenon of jaggy may occur if the angle of an edge of an image is close to the angle of halftone dots so that the edge interferes with the halftone dots.
The above-described phenomena may become more visible as the screen ruling of halftone dots in AM screen processing becomes low. In order to solve the above-described problem, a conventional method switches between AM screen processing and FM screen processing according to the state of an image region.
More specifically, the conventional method uses the FM screen processing, instead of using the AM screen processing, for an image region in which document moire, broken thin line, or jaggy occurs. However, the FM screen processing has a gradation characteristic different from a gradation characteristic of the AM screen processing.
In the FM screen processing, dots in a highlight region may not be more concentrated than dots in AM screen processing are. Accordingly, more dots may be lost in the case of FM screen processing than in AM screen processing. As a result, the density of a highlight region may become low in FM screen processing.
Furthermore, in FM screen processing, dots are more highly dispersed in a range from a halftone region to a shadow region compared with the dots processed by the AM screen processing. Accordingly, the gain of dots may become high. As a result, in the FM screen processing, the density may become high in the range from the halftone region to the shadow region.
Accordingly, if the type of screen processing is changed according to which image region is to be processed, the density may vary for each image region. In order to solve the above-described problem, Japanese Patent Application Laid-Open No. 07-333822 discusses a method that uses different gradation correction tables for regions processed by different types of screen processing.
With the method discussed in Japanese Patent Application Laid-Open No. 07-333822, the region processed by FM screen processing and the region processed by AM screen processing can have the same gradation characteristic. However, according to the method discussed in Japanese Patent Application Laid-Open No. 07-333822, the following problem may arise.
A gradation correction table is generated by color-measuring a uniform patch image which does not have a spatial frequency component except direct current (DC) components. Accordingly, gradation correction can be executed on a uniform image region with high accuracy. On the other hand, the gradation of an image region whose spatial frequency is high cannot be always appropriately corrected.
The gradation characteristic may vary according to the type of screen processing to be executed and the spatial frequency of an input image. In an image region whose spatial frequency is high, dots may likely to be highly dispersed and the gain of dots may easily become high. Accordingly, the density of the high-spatial frequency image region may more easily become high compared with an image region whose spatial frequency is low.
Accordingly, if the same gradation correction table is used in processing image regions of different spatial frequencies, a target density cannot be reproduced for a high-spatial frequency image region.